The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus for multi-stage processing of channels of acquired MR signals to improve signal-to-noise ratio (SNR) without compromising field-of-view (FOV) for simultaneous data acquisition from an array of RF coils.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
For localized high-resolution MR imaging applications, an array of surface RF coils are typically used because a smaller or more localized RF coil has a higher B1 field and less loading-induced noise, which yields a higher intrinsic signal-to-noise ratio (SNR) for the receiver or coil. Generally, the following expression:
                    SNR        ≈                              V            ⁢                                                  ⁢                          ω              2                        ⁢            B1                                              4              ⁢              k              ⁢                                                          ⁢              T              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              f              ⁢                                                          ⁢              R                                                                    (                      Eqn            .                                                  ⁢            1                    )                ;            where V is the sample volume, Δƒ is the receiver bandwidth, R is the total noise, may be used to define or quantify the SNR associated with a particular RF coil or receiver of an RF receiver coil array or system. As one skilled in the art will appreciate, as SNR decreases, the diagnostic value of the final reconstructed image also decreases. That is, as SNR decreases, resolution of the final reconstructed image decreases.
Current signal processing techniques support the combination of signals from individual coils or receivers into combo- or super-coils. This combination of coils effectively allows signals to be received from a desired FOV or area that is larger than that individually supported by a particular coil. The optimal area or volume supported by a particular coil depends, in large part, on the size of the coil. For instance, coil sensitivity decreases with increasing coil size. On the other hand, reducing coil size increases the total number of coils needed to receive signals from the desired FOV.
For any given application, there is an optimized coil size. To this end, using an oversized coil alone will result in intrinsic and undesirable SNR penalty. Quadrature analog combination is a signal processing technique that has been developed to address this SNR penalty associated with oversized coils, but with some limitations. Quadrature analog combination can provide √{square root over (2)} times higher SNR where the B1 field from each coil are orthogonal to each other. The B1 field between two quadrature coils, however, is not always orthogonal and, as such, the direction of B1 field between the coils may vary from place to place. Thus, the combined SNR can be worse than the respective SNR of the individual coils. This worsening of SNR may be attributed to phase cancellation. In this regard, the combined SNR does not yield consistent SNR improvement over the entire FOV. Furthermore, larger sized coils present more isolation problems between coil elements from the stronger inductive coupling therebetween which will inherently degrade the combined SNR. Therefore, the benefit of using a large quadrature pair of coils with quadrature analog combining to improve SNR over a large FOV is limited.
In contrast to analog combination, due to the fact that digital combination only combines the magnitude of each signal regardless of the phase between the received signals, the digital combination of signals received by independent receivers can be used to provide net gain of SNR instead of worsening of SNR over the desired and enlarged FOV. Accordingly, RF coils could be designed such that each coil element has an optimized size for B1 field penetration and utilize the MR system independent receivers to yield the desired FOV, provided that there are enough number of MR system independent receivers.
However, MR scanners are subject to a limited number of receivers and, as such, a tradeoff of designing coils having desirable FOV and designing coils having optimized SNR must be made.
It would, therefore, be desirable to have a method capable of combining MR signals received from an array of receiver coils so as to realize optimized SNR without compromising FOV for simultaneous MR data acquisition by the array.